The present invention relates to improvements in systems for the measurement of electrolytic conductivity. Typically, these systems are used for the measurement of the conductivity of water and aqueous or non-aqueous solutions in environmental, industrial, medical, and other applications where an indication of the total ionic content of the sample liquid is desired. A typical system consists of a conductivity cell and meter. The meter applies an electrical input signal (generally a controlled voltage signal) to the cell which contacts the sample. The meter also senses a resultant output signal from the cell (generally a current signal) that is usually a linear function of the solution conductivity and a property of the cell called the cell constant.
There are many types of cells and meters. Typical cells may have two or four electrodes and may contain integral temperature sensors; cells may be dipped into solutions, solutions may be placed into cells, or solutions may flow through cells. Various meters may apply excitation signals differing in voltage, frequency, and waveform; they may display results with a digital or analog display; they may sense and display temperature; and they may perform various calculations in order to compensate automatically for the effect of temperature on the conductivity of various samples.
In order that the ensuing description be clearly understood, the following definitions are provided:
Conductivity is a bulk property of a material and represents quantitatively the capacity of that material to conduct electricity; its reciprocal is resistivity. Conductivity does not depend on the quantity, shape, or size of the conducting material. Earlier literature often uses the terms specific conductance and its reciprocal specific resistance in place of conductivity and resistivity. Common units of conductivity are siemens/cm (S/cm), millisiemens/cm (mS/cm), and microsiemens/cm (.mu.S/cm). PA1 Conductance is a property of a particular piece of material, i.e., a component or device. Its reciprocal is resistance. As an illustration of the difference between conductance and conductivity, consider two different pieces or pure copper wire: they may have different conductance values but will always have the same conductivity of the material, the conductivity of copper. Common units of conductance are siemens (S), millisiemens (mS), and microsiemens (.mu.S). PA1 The Cell Constant of an electrolytic conductivity cell is a conversion factor which converts conductance to conductivity. It has units of cm.sup.-1. The cell constant is a function of cell geometry. In a two-electrode conductivity cell, the cell constant is a predictable function of the area of the electrodes and the distance between them, if the electrodes are non-polarized. It is useful to note that when the cell constant equals 1.0, the solution conductivity is equal to the cell conductance. PA1 Polarization is a condition in which a resistance, not determined by the bulk resistance of the solution under test, exists at the interface between an electrode and the test solution. This is an undesirable phenomenon which causes errors in conductivity measurements. Conductivity cell and meter designers enact measures which eliminate or minimize electrode polarization. For example, on the meter side, AC excitation signals are virtually always used. This minimizes alteration of the ionic concentrations in the solution in the vicinity of the electrode surfaces. On the electrode side, materials such as platinum black are used. The catalytic nature of the platinum black surface facilitates electron transfer between the electrode and the solution. PA1 Built-in display of a parameter that indicates the degree of non-linearity; PA1 A means of automatically recognizing specific standardizing solutions; and PA1 A temperature-compensated algorithm for determining when a conductivity measurement signal is stable.
Generally, a linear range of operation is specified for a conductivity system. This is the range of sample conductivity over which the cell output signal is linearly proportional to solution conductivity. Or, expressed differently, the linear range is the range over which the cell constant is truly a constant.
Conductivity systems must be calibrated. It is typical that the conductivity meter is calibrated at the point of manufacture and is able to convert the cell output signal to conductance without further calibration by the user, though he or she may wish to verify meter calibration using a standard resistor in place of the cell. The user must however carry out a procedure which serves to calibrate the conductivity cell. This is equivalent to determining the cell constant.
In the cell calibration step, the user places the cell in contact with a solution of known conductivity, often called a standard solution or simply a standard. The meter is then adjusted to display the known conductivity of that solution at the temperature of the measurement; alternatively, a cell constant value can be input into the meter such that the correct conductivity reading is displayed. This process is equivalent to defining a calibration curve; in this case, the curve is assumed to be a straight line. A line is defined by two points: here, one point is obtained through the assumption that an output signal of zero equals a conductivity of zero, and the second point is obtained in the calibration step. The calibration line is defined by the equation: EQU C=k*S.sub.raw
Where c is the conductivity in appropriate units such as mS/cm, k is the cell constant and has units of cm.sup.-1, and S.sub.Raw is the raw cell output signal in units of mS. This equation illustrates the role of the cell constant in converting the sample conductance value, S.sub.raw into a conductivity value, c.
Linearity of a conductivity system is an ideal. Typically, there is a range of sample conductivity values over which satisfactory linearity is observed for a given system. At the low end of the range, deviations from linearity can be ascribed to such factors as capacitive impedance, which in an AC measurement, increases at high resistances, i.e., low conductivities. At the high end of the conductivity range, the resistances of wiring and other components in measuring circuit become significant compared to the resistance of the cell. Also, at the high end of the range, the current through the cell is greater: this increases the tendency of the electrodes to become polarized.